Semiconductor devices are known, for example as described in EP-A-243953, which comprise a semiconductor body having a substrate on which is provided a channel-defining region extending between input and output regions, generally source and drain regions, the channel-defining region comprising a channel layer forming a heterojection with at least one barrier layer so as to form within the channel layer a two-dimensional free charge carrier gas of one conductivity type for providing between the input and output regions a conduction channel controllable by a gate electrode overlying the channel-defining region.
EP-A-243953 is concerned with a semiconductor device such as an FET or HEMT in which the channel-defining region is constructed so as to reduce the variation of electron mobility with the strength of the lateral electric field existing in the channel-defining region. In particular, the channel-defining region is formed by a series of alternate thin layers of two different materials one of which is of larger band gap and so defines barrier layers forming heterojunctions with the other layers which provide, under low electric fields, channels layers in which a two dimensional electron gas is provided. The two materials are selected such that the electron mobility in the one material (normally the barrier-defining material) is lower than the electron mobility in the other material under low applied electric fields but is higher under high electric fields. Thus, conduction occurs preferentially in the channel layers defined by the other material under low electric fields and in the barrier layers defined by the one material under high electric fields. Accordingly the dependency of electron mobility on electric field can be reduced. However, other problems arise when such semiconductor devices are subjected to high electric fields. In particular, the existence of a high lateral electric field in the channel-defining region tends to cause hot charge carriers (that is charge carriers which are not in thermal equilibrium with the crystal lattice) to be emitted from the channel-defining region towards the substrate. The ejection of such hot charge carriers into the substrate is the main origin of the finite output impedance characteristics of FETs and HEMTs.